Uranium occurs naturally on earth. We mine it, refine it, and use it as fuel in nuclear reactors. All other elements that can sustain the nuclear fission chain reaction, including plutonium, do not occur naturally on earth*. The only way to obtain plutonium is to produce it from other elements through neutron radiation. Neutrons in a nuclear reactor produce power through the fission of uranium, but they also convert some of the uranium into plutonium. Thus, all uranium-fueled reactors contain at least some plutonium.

After spent fuel is removed from a reactor, the Japanese nuclear industry sometimes reprocesses it by separating the plutonium and uranium from the fission products, which are (useless) light elements that arise when uranium splits in the fission reaction. The separated plutonium and uranium are often mixed to make new “fresh” fuel that has a higher relative plutonium concentration than the original spent fuel. This is called mixed oxide (MOX) fuel, as the uranium and plutonium are ceramics in oxide chemical form (UO2 and PuO2). Uranium-fueled reactors also typically use the oxide form of uranium (UO2). It is crucial to understand that uranium oxide fuel contains plutonium just as MOX fuel does. The only difference is the relative concentrations of the two elements. MOX fuel is one way in which the nuclear industry can “recycle” nuclear fuel in order to use the earth’s uranium resources more efficiently and responsibly.

*As a side note, the element thorium occurs naturally on earth and can be used to produce uranium, although it cannot sustain the fission chain reaction itself.